1. Field of the Invention
The present invention relates to a rate adaptive pacemaker of the type having circuitry for determining the demand of the patient's organism, a pacing rate controller for controlling the pacing rate in response to the patient's demands and a pacing rate limiter for preventing the pacing rate from becoming too high.
2. Description of the Prior Art
Pacing rates that are too high can appear in a rate adaptive pacemaker due to the physical demand of the patient's organism and heart. This may cause lack of oxygen supply to the myocardium. Thus, in certain conditions the heart may not be able to satisfy the physiological needs of the patient's organism and heart if the pacing rate is not limited.
Several different proposals for upwardly limiting the pacing rate have been presented. Thus in e.g. U.S. Pat. No. 5,350,409 a rate adaptive pacemaker is described having an upper pacing limit programmed beyond which rate the pacemaker will not generate and deliver stimulation pulses. U.S. Pat. No. 5,792,195 discloses an acceleration sensed safe upper rate envelope for calculating the hemodynamic upper rate limit for a rate adaptive pacemaker. From the output signal from an accelerometer the time of occurrence of a specific heart sound in relation to a previously occurring ventricular depolarization event is then derived and this heart sound information is used to establish a hemodynamic upper rate limit for the pacemaker. Also European Application 0 879 618 describes a rate modulated heart stimulator having a programmable maximum sensor rate. This heart stimulator also includes an ischemia detector and in response to the detection of an ischemia the maximum allowable stimulation rate is decreased.
The limit values are determined from patients' diagnosis and the setting can be either constant or externally programable.